PETORLOGICAL AND CHEMICAL CHARACTERISTICS OF LIPTINITE-RICH COALS FROM ALBERTA, CANADA

A suite of liptinite-rich coals from the Lower Cretaceous Mannville Group in the subsurface of Alberta, Canada has been studied with respect to its organic petrology and organic geochemistry. The samples are rich in fluorescing vitrinite (perhydrous), sporinite, cutinite, exsudatinite and bituminite. Rock-Eval analysis shows that the coals are enriched in hydrogen (hydrogen index up to 320 mg HC/g TOC) and depleted in oxygen (oxygen index up to 11 mg CO2/g TOC). The value of T(max) ranges from 445 to 460-degrees-C, indicating a late catagenic level of thermal maturity (equivalent to %Ro = 1.0-1.1), but the vitrinite reflectance is between 0.68 and 0.81%, indicating that it is lowered by as much as 0.4%. The pyrolysis chromatograms of the coal samples are dominated by substituted benzenes (toluene and xylenes), naphthalenes, phenanthrenes, substituted phenols (cresols) and small amounts of n-alkanes in the range C20+. Gas chromatography-mass spectrometry of the steranes and terpanes indicates little or no contribution of allochthonous bitumen to the coals. In addition, the m/z = 217 sterane traces are dominated by C29 compounds with little C27 and C28; this, along with the high terpane to sterane ratio (m/z 191 to m/z 217 = 8.8) indicates that the bitumen was generated from terrestrially derived organic matter. The bitumen in the coals has most likely been autogenerated by the coals themselves in response to thermal stress. The originally sapropelic coals were probably deposited in an anaerobic environment and were much enriched in hydrogen. This study shows that terrigenous organic matter, when associated with an anomalous liptinite content, may have the potential to generate bitumen.